In brake boosters of a type disclosed in U.S. Pat. No. 4,953,446, pressure differential is created across a wall that separates a vacuum chamber from a control chamber by air communicated from a passenger compartment. The pressure differential acts on the wall to develop an output force corresponding to an input force applied to a control valve. The control valve includes a return spring that urges a plunger toward an atmospheric seat on a poppet member and a seat spring to urge the poppet toward a vacuum seat. The input force is applied to the push rod which compresses the return spring to move the plunger and sequentially allow the seat spring to seat the poppet member on the vacuum seat and the plunger to move away from the poppet member to thereafter allow air to flow to the control chamber. The communication of air through the poppet can create noise during the development of a pressure differential.
In analyzing the operation of brake boosters it was observed that air supplied to a control valve most often is obtained from the passenger compartment of a vehicle. If the sounds caused by the flow of air from the passenger compartment during the operation of a boost unit could be shifted from the passenger compartment to an engine compartment, a passenger would not hear the booster operation. Unfortunately space in the engine compartment is limited and relocating the brake booster within the engine compartment of a vehicle still requires at least a portion of the components that are associated with a brake booster to remain in a passenger compartment of the vehicle.
U.S. Pat. No. 5,487,324 discloses structure whereby air from the engine compartment may be utilized and supplied to a control chamber of a boost unit to develop the pressure differential during a brake application and U.S. Pat. No. 6,073,535 discloses a bracket member with a resonant chamber whereby noise is created in the engine compartment is attenuated in a resonant chamber. While the structure in U.S. Pat. No. 6,073,535 functions in an adequate manner when a offset distance is sufficient to provide for the inclusion of a resonant chamber it often happens the under hood space available for the boost unit may be limited by other structural components in the engine compartment. U.S. patent application 10/708,246 discloses a spacer that positions a boost unit away from a dash panel that is mated with a boot to define and maintain a substantially uniform flow path through which air from the engine compartment is presented to a control valve in the boost unit to development of an output force in the boost unit to effect a brake application. This valve functions in an adequate manner however noise created in the engine compartment may be heard in the passenger compartment.